Electrochemical cells are widely employed for electrochemical and biological applications. A typical electrochemical cell has a working electrode, a counter electrode and a non-current carrying reference electrode. Controlling and measuring the electrical parameters of an electrode reaction in a cell is achieved by potential, current and charge control methods.
Thermodynamically favorable electrochemical systems are illustrated by fuel cells and batteries. A fuel cell involves an electrogenerative mode of operation. U.S. Pat. No. 3,147,203 describes an electrochemical system comprising separate anodic and cathodic reaction zones containing an anode and cathode connected through an external circuit, and the zones are connected by a salt bridge or a semi-permeable membrane. The described cell is suitable for electrogenerative partial oxidation of hydrocarbons. For example, olefin feed is introduced into the anodic reaction zone and oxygen is introduced into the cathodic reaction zone, with the result that the olefin is oxidized to an aldehyde or ketone and electric current is generated.
Generally, an electrogenerative process is a coupling of suitable electrochemical reactions at opposing electrodes, separated by an electrolyte barrier to yield a desired chemical product with the generation of low voltage electrical energy as a byproduct.
The current (rate of reaction) is controlled by an external load resistor. The anodic and cathodic potentials are functions of the current by the following simplified equations: EQU E.sub.a =E.sub.a.sup.o +b.sub.1 log i.sub.a EQU E.sub.c =E.sub.c.sup.o -b.sub.2 log i.sub.c
E.sub.a.sup.o and E.sub.c.sup.o are reversible potentials, b.sub.1 and b.sub.2 are kinetic parameters, and i.sub.a and i.sub.c are the anodic and cathodic current densities.
In accordance with these equations, as more current is allowed to pass (lower resistance) the anodic potential increases and the cathodic potential decreases. Since the potential at which the electrode operates determines the reaction which takes place (e.g., partial oxidation), rigorous control of this potential is essential in order to control the reaction selectivity.
For example (with reference to FIG. 1), assume that a reaction is taking place electrogeneratively and reactant R.sub.1 is being oxidized to some species O'. The potential of the anode is controlled by the current density, i.sub.a, and the reaction kinetics, b.sub.1. If the current increases or if the kinetics change due to electrode fouling or changes in the reactant concentration, the potential of the anode will increase, possibly to the point where O' is further oxidized to O". If one wishes to produce O' selectively, the potential of the anode must stay below the reversible potential for its oxidation to O".
Control of potential in an electrochemical system of the type described above usually is achieved by operating at a constant current, with the need that the kinetics remain constant so that a constant potential is maintained. In essence, the potential is indirectly controlled by controlling the current. In practice, this indirect method of controlling the half-cell potential in a thermodynamically favorable electrochemical system is unsatisfactory.
Accordingly, it is an object of this invention to provide a novel passive potentiostat apparatus which is adapted for control of the half-cell potential of an operating thermodynamically favorable electrochemical cell.
It is another object of this invention to provide a dynamic system for controlling the anodic potential of an electrogenerative process.
Other objects and advantages of the present invention shall become apparent from the accompanying description and drawings.